1. Field of the Invention
The present invention relates to a method of forming a tunnel oxide film in a semiconductor device and, more particularly, to a method of forming a tunnel oxide film in a semiconductor device capable of preventing degradation of film quality caused by absorbed carbon components and a rough surface on a semiconductor substrate.
2. Discussion of Related Art
FIGS. 1A to 1D are cross-sectional views for explaining a conventional method of forming a tunnel oxide film in a semiconductor device.
Referring to FIG. 1A, in order to form a gate oxide film for a high voltage transistor, a first oxide film 2 having a thickness of 350 Å is formed on a semiconductor substrate 1 which has undergone predetermined processes.
Referring to FIG. 1B, a photo resist film 3 is formed on the first oxide film 2. Then, the photo resist film 3 is patterned to expose the first oxide film 2 formed on portions in which memory cells and low voltage transistors are to be formed.
Referring to FIG. 1C, the exposed portions on the first oxide film 2 are removed after the patterned photo resist film 3 is consolidated by a Descum process. In this case, the first oxide film 2 is removed by using a 300:1 BOE (Buffered Oxide Etchant) for a period of 2,280 seconds, and then the photo resist film 3 is removed by using H2SO4 solution. Subsequently, a cleaning process is performed by using an SC-1 solution.
Referring to FIG. 1D, a second oxide film 4 having a thickness of 80 Å is formed on a whole top surface. In this case, a thick gate oxide film including the first oxide film 2 and the second oxide film 4 is formed in a high voltage transistor area, whereas a tunnel oxide film including the second oxide film 4 is formed in a memory cell area and a low voltage transistor area.
According to the conventional method, during the removal of the first oxide film 2 in a memory cell area and a low voltage transistor area by using a BOE, an over-etching of 30% occurs so that surface roughness of the semiconductor substrate 1 is increased and carbon components included in the photo resist film are absorbed on the surface of the semiconductor substrate 1 when the photo resist film 3 is removed by using H2SO4 solution. The absorbed carbon components are seldom removed even by a subsequent cleaning process using an SC-1 solution or a preliminary cleaning process using a 50:1 HF solution before the tunnel oxide film is formed. As a result, the remaining carbon components bring about silicon dangling bonds. This makes film quality be degraded when a tunnel oxide film is formed under the state that the carbon components exist, whereby the resulted device may have a bad electrical property.
FIGS. 2 and 3 are graphs showing the results of a constant current stress test (CCST) for a flash memory device according to the conventional method. The results show that uniformity of distribution is bad and a proportion of defect is also high. Particularly, most of initial defects are distributed in the edge portion of a wafer.